Electrical discharge machining (EDM) is a process by which an electrically conductive metallic workpiece can be machined by selective removal of the metallic workpiece material through an electrical spark discharged from an electrode. This process is used to form a variety of complex shapes in materials that would otherwise be difficult to obtain by drilling, milling, grinding, or other conventional cutting techniques.
An electrode blank, from which an electrode may be formed, can be fixed in position on an electrode holder in a variety of ways. Some of the more common methods used are to solder the electrode blank on the electrode holder, use of a press fit and glue joint for holding an electrode blank on an electrode holder, or screwing an electrode blank onto an electrode holder with a glue joint. The electrode blank may then be machined into an electrode of a particular configuration for use in an EDM process. The electrode holders, with the electrodes mounted thereon, are then mounted in an EDM machine for carrying out an EDM process.
In some instances, a series of electrode holders, for example, up to 16 electrode holders, may be mounted in a 3R-SP50223 Robomatic 16 VS Automatic Electrode Changer, available from System 3R U.S.A., Inc., Cedar Grove, New Jersey. A plurality of specifically configured electrodes are mounted on these electrode holders, which are then mounted in the EDM machine and successively applied to a workpiece to produce a specifically configured end product. It is imperative that the electrode holder and the electrode be accurately positioned along all three reference planes, x, y, and z, to accurately machine a product to a desired configuration.
In some instances, it is not practical to configure an electrode from an electrode blank that is first mounted on an individual electrode holder because the number of electrode holders required is cost prohibitive. Small- to medium-size, square, or rectangular electrodes are sometimes used in large quantities, and it is more cost efficient to purchase precision-ground graphite electrode blanks. The required detail is then machined on the working end of the graphite blank, while the blank is held in a grinding vice or V-block. The finished electrode is then transferred to a similar V-block mounted on the ram or quill of an EDM machine.
This process, while convenient, presents problems in predicting the accurate location of each electrode in reference to the electrode holder and thereby to the workpiece. Usually, the only method of locating the position of the electrode is by use of an electrical edge finder. These edge finders have proven to be unreliable and useful only when broad tolerances are acceptable. Aligning a flushing hole through the center of the electrode with the flushing hole extending through the center of the electrode holder also becomes complicated by the present practices and, in many instances, often proves impossible.
In U.S. Pat. No. 4,583,432 to Bricker, a supercentering electrode holder is described, which supercenters an electrode relative to a concentrically rotatable cylinder. A clamping subassembly is centerable with respect to a centering member. The centering member has a centering projection, which is supercenterable with respect to a holder by several axially normal adjustment screws engaging side surfaces of the centering projection. The cylinder is slowly rotated, thereby rotating the tooling and the clamped electrode. The eccentricity of rotation is thereby determined, and appropriate, incremental adjustments are made using the adjustment screws. This process is repeated until no more adjustments are needed to remove the eccentricity of rotation.
In U.S. Pat. Nos. 2,472,040 and 3,323,809 to Brookfield, a work holder with a V-block is disclosed for clamping an electrode. Adjustment of the electrode is achieved by releasing set screws, moving the work holder, and resetting the set screws. Repeated adjustments are made until the workpiece is centered.
In the patents to Brookfield and Bricker, a series of tightening and loosening adjustments must be made in order to adjust a clamping assembly with respect to a cylindrical electrode holder. These series of adjustments are time-consuming and fail to ensure an immediate supercentering of an electrode which is accurate with respect to a work holder.
Further examples of clamping subassemblies which initially clamp an electrode in a tool followed by several adjustments to position the electrode relative to the tool holder are found in U.S. Pat. Nos. 3,094,821 and 2,449,459 to Eckert, U.S. Pat. No. 3,474,215 to Johanson, and U.S. Pat. No. 4,655,654 to Portas.
In the present invention, the problems of the prior art are overcome by the immediate and automatic supercentering of an electrode with respect to an electrode holder having machined flats extending perpendicular to each other and perpendicular to an electrode holder end face.